pasi hakonen firearm suppressors – structures and alternative

PASI HAKONEN
FIREARM SUPPRESSORS – STRUCTURES AND ALTERNATIVE
MATERIALS
Master of Science Thesis
Examiner: Professor Tuomo Tiainen
Examiner and topic was approved in
the Faculty of Automation,
Mechanical and Material
Engineering council meeting in Nov
3rd, 2010
II
TIIVISTELMÄ
TAMPEREEN TEKNILLINEN YLIOPISTO
Materiaalitekniikan koulutusohjelma
HAKONEN, PASI: Äänenvaimentimet, rakenteet ja vaihtoehtoiset materiaalit
Diplomityö, 47 sivua
Lokakuu 2011
Pääaine: Metallimateriaalit
Tarkastaja: professori Tuomo Tiainen
Avainsanat: Äänenvaimennin, kuulovaurio, kivääri, kuulosuojaus
Joka vuosi lukuisat metsästäjät, ampumaharrastajat ja varusmiehet kärsivät tinnituksesta
ja eriasteisista kuulovaurioista. Suuri osa näistä ongelmista johtuu käsiaseiden
laukausäänille altistumisesta ja epäonnistuneesta kuulonsuojauksesta ja ne olisivat
vältettävissä asianmukaisilla aseisiin asennettavilla äänenvaimentimilla.
Painava teräsvaimennin voi tehdä aseesta kömpelön käsiteltävän ja raskaan
kantaa pitkiä matkoja, mikä voi osaltaan selittää sen miksi vaimentimien käyttö ei ole
yleistynyt vielä kattavaksi. Tässä diplomityössä pyritään löytämään tähän ongelmaan
ratkaisu
kevyempien
materiaalivaihtoehtojen
avulla.
Tutkimus
alkoi
kirjallisuusselvityksellä, jonka pohjalta suoritettiin materiaalivalinta sopivien
vaihtoehtoisten materiaalien löytämiseksi. Löydetyistä materiaalivaihtoehdoista valittiin
kiinnostavimmat, joista valmistettiin prototyyppi testiammuntoihin.
Ensimmäinen testi keskittyi hiilikuitukomposiitin soveltuvuuteen vaimentimen
ulkokuoreksi. Testissä havaittiin materiaalin kestävän huomattavasti suurempia
lämpökuormia kuin alkuperäiset ennakkokäsitykset ja odotukset antoivat ymmärtää.
Hiilikuitukomposiittivaimennin halkesi vasta kestettyään huomattavasti vaativampia
olosuhteita kuin mihin kyseinen vaimenninprototyyppi oli tarkoitettu. Testin tulokset
olivat lupaavia hiilikuidun soveltuvuuden suhteen mutta aikaongelmat estivät
perusteellisen testaamisen ja lopulliset tulokset jäivät suuntaa-antaviksi. Prototyyppien
rakentaminen ja testaaminen olivat vielä diplomityön kirjoittamisen aikana kesken ja ne
jäivät odottamaan sopivaa testaamisajankohtaa.
Tutkimuksen selkein lopputulos on hiilikuitukomposiittien odotuksia parempi
kestävyys äänenvaimentimissa. Lisätutkimusta tarvitaan hiilikuitukomposiittien ja
anodisoidun alumiinin yhdistämisestä kiväärikaliiberisten äänenvaimentimien
rakenteessa.
III
ABSTRACT
TAMPERE UNIVERSITY OF TECHNOLOGY
Master’s Degree Programme in Materials Engineering
HAKONEN, PASI: Firearm suppressors – structures and alternative materials
Master of Science Thesis, 47 pages
October 2011
Major: Metallic materials
Examiner: Professor Tuomo Tiainen
Keywords: Firearm, suppressor, silencer, hearing protection, hearing damage
Each year several hunters, conscripts and sport shooters suffer from tinnitus and other
problems with their hearing. A great part of these problems are resulting from the
exposure to noise from small arms fire without adequate hearing protection. Many of
these hearing problems could have been avoided with the proper use of fire arm
suppressors.
A heavy steel suppressor may turn a rifle unwieldy and uncomfortable to carry and
therefore less tempting to use. This thesis aimed for solving this problem by exploring
lighter alternative materials for suppressor use. The research began with a literary study
on the subject and on that basis it was continued by exploring possible alternative
materials with a computer aided selection process. These materials were tested by
building a prototype suppressor and by testing the material suitability with real fire
testing.
The carbon fiber composites which were subjected to first test proved to withstand
heat to much greater extent that what was originally expected. The material ruptured
only after considerably heavier exposure than the suppressor was intended to endure.
The results of the tests showed promise for the carbon fiber composites, but they failed
to provide conclusive evidence as timing problems prevented extensive testing. The
fabrication and prototype testing processes are still underway and they wait for future
testing possibilities.
The main conclusion from this study is that future research and testing are still
needed to verify the suitability of the combination of carbon fiber composite and
anodized aluminum in rifle caliber suppressors. The preliminary results are very
promising.
IV
PREFACE
This thesis is written on the basis of the research on firearm suppressors conducted in
Tampere University of Technology Department of Materials Science, in behalf of
Oricopa Oy. The aim of the research was to improve already existing products with
lighter material alternatives. This thesis contains theoretical background on firearm
suppressors and on the computer-based process of finding alternative materials. The
design process from first prototypes to future design possibilities is also described in the
thesis. This document is intended as a basis for further research into the subject and to
provide insight from material engineering perspective.
I would like to thank my professor Tuomo Tiainen for his valuable input and
expertise and my instructor Mikko Nieminen for his reflection and for the endless
supply of ideas for the research project. I would also like to acknowledge the personnel
of Oricopa Oy, especially Taito Martiskainen and Vesa Saviahde for their insight and
knowledge on the subject.
Writing this thesis would have never been possible without my family who has
supported me through my studies and I want to thank them for all they have done and
said. Still the most important person through this whole ordeal has been my wife Maija
who found new ways to inspire me and helped me where I would not have had the
strength to continue alone.
V
TABLE OF CONTENTS
1.
2.
Introduction ............................................................................................................... 1
Principles of sound suppression ................................................................................ 2
2.1. Reasons for sound suppressors.......................................................................... 4
2.1.1.
Health issues ..................................................................................... 4
2.1.2.
Hunting.............................................................................................. 4
2.1.3.
Military service ................................................................................. 5
2.2. Noise sources .................................................................................................... 5
2.2.1.
Muzzle report .................................................................................... 6
2.2.2.
Ballistic crack .................................................................................... 8
2.2.3.
Other sound sources .......................................................................... 8
2.3. Elimination of noise .......................................................................................... 9
2.3.1.
Protective equipment ......................................................................... 9
2.3.2.
Protection of the environment ......................................................... 10
2.3.3.
Noise damping structures in shooting ranges.................................. 11
2.3.4.
Industrial sound suppressors ........................................................... 13
2.3.5.
Suppressor structure elements ......................................................... 14
The aim of the work ........................................................................................................ 16
3. Suppressor prototypes ............................................................................................. 17
3.1. Structures and designs ..................................................................................... 17
3.1.1.
Initial structure ................................................................................ 21
3.1.2.
First prototype ................................................................................. 21
3.1.3.
The second prototype ...................................................................... 22
3.1.4.
Future prototypes ............................................................................ 24
3.2. Materials .......................................................................................................... 24
3.2.1.
Material requirements ..................................................................... 24
3.2.2.
Materials used in suppressors.......................................................... 25
3.2.3.
Search for alternative materials ....................................................... 27
4. Prototype testing ..................................................................................................... 36
4.1. Testing equipment ........................................................................................... 38
4.2. Suppressor test ................................................................................................ 39
4.2.1.
Real fire test .................................................................................... 39
5. Results and discussion ............................................................................................ 42
6. Conclusions ............................................................................................................. 44
Bibliography.................................................................................................................... 45
VI
ABRREVIATIONS AND NOTATION
A-duration
First round pop
Baffle
Ballistic crack
Caliber
Centerfire
CES
Inconel
Muzzle blast
Muzzle brake
Muzzle report
Reflex suppressor
Rimfire
Rise time
Silencer
SPL
Suppressor
Zeroing a weapon
The time between the beginning of impulsive pressure
increase and the return of the pressure to the ambient level.
A phenomenon with certain suppressor types where the
unburnt gunpowder ignites within the suppressor during
first discharge.
A baffle is a structural part in a suppressor that guides the
expanding propellant gas to slow the gas discharge from the
suppressor.
A projectile travelling at supersonic velocities produces a
loud noise during flight. This is known as ballistic crack.
Caliber refers to the diameter of the bore of a firearm.
Caliber can either be announced in fractions of an inch
(.308 and .50) or in millimeters (7.62 and 12.7,
correspondingly).
A cartridge type where the primer is located in the center of
the cartridge base. A high pressure cartridge used in most of
the modern firearms.
Cambridge Engineering Selector is a software designed to
aid the process of finding possible materials and
manufacturing processes for different applications.
A registered trademark of Special Metals Corporation. A
family of nickel-chromium superalloys.
Alternative name for muzzle report.
A device installed to the muzzle of a weapon that diverts
and redirects propellant gas backwards to reduce the recoil.
The sound caused by expanding propellant gas.
A suppressor design where a significant part of the
suppressor extends back over the muzzle to form a big
primary expansion chamber.
A low pressure cartridge type used in some pistols and
small caliber rifles.
A property of a sound level measuring device that describes
how quickly the device reacts to sound pressure changes.
A device usually attached to the muzzle of a gun to suppress
the muzzle report.
Sound pressure level, measured in decibels, describes the
pressure variation caused by a sound source.
A more formal term used of weapon silencers.
Calibration of weapon sights and optics.
1
1.
INTRODUCTION
Anyone who has experienced tinnitus knows how annoying it can be, especially if it is a
constant nuisance. Hunters and recreational shooters are familiar with hearing problems
as a study shows that every 5 years of recreational shooting increases chances of
permanent hearing damage by 7% [1]. Hunting requires constant awareness of the
surroundings and hearing protectors are easily left out to maintain situational awareness.
This easily explains why older shooters may have problems with their hearing. In cases
like hunting where earplugs and other hearing protectors are cumbersome other ways to
protect the hearing of shooters and observers becomes increasingly important. Firearm
suppressors offer a possible solution to such situations.
The most important aspect in the common civilian usage of suppressors is the
protection of the hearing of the weapon operator and anyone in the vicinity. As the
muzzle report of a rifle caliber weapon may exceed sound pressure levels of over 160
dB in the vicinity of the weapon, it poses a clear health hazard to the hearing of the
shooter and everyone else in the proximity. With rifle caliber weapons a suppressor can
be used to decrease the peak sound levels to 140 dB and below, which is considered as a
limit when an impulsive sound produces hearing damage [2]. The suppressor also works
in other ways besides than just damping the muzzle report. It influences the shooting
experience and even enhances accuracy in some cases.
Suppressors provide an excellent way to protect the surroundings of shooting
ranges from noise pollution. According to reference [2], the noise damping with
structural changes in the shooting range requires significantly bigger investments as
compared to the damping provided by suppressors. These facts alone already pose a
valuable reason for the use and development of suppressors for civilian and military
service. The internal structure of suppressors has been a noticeable target for research in
history and there have been clear advancements in efficiency and size. [3]
In contrast alternative materials for suppressors have been studied less and the
materials used in silencers rely heavily on traditional steels as the usage circumstances
set significant requirements on the materials. The flow speed of air near the gun barrel
can exceed speed of sound by a factor of three, while temperature can rise as high as
1000°C and the gas pressure can achieve levels from 200 to 600 times the atmospheric
pressure. [4] These circumstances set great demands on the used materials. However,
some research work has been carried out also on materials and as a result titanium, high
temperature superalloys and carbon fiber composites have been used in modern
suppressors. [3]
2
2.
PRINCIPLES OF SOUND SUPPRESSION
In order to understand the methods used to suppress sound, one must first understand
what sound is. Sound has been described as pressure, density and temperature variations
which progress through a medium in waves. The wave characteristic of sound is
responsible for such phenomena as scattering, absorption, refraction, interference,
diffraction and reflection which play major role in the way sound behaves. The speed of
sound is largely dependent on the sound medium where the waves propagate and on the
surrounding conditions. Temperature, pressure and other outside factors influence the
interactions between sound waves and particles. [5]
In order to suppress a sound, several methods can be used. Placing obstacles in
the path of sound waves, using absorbing materials, specific structures and even
electronic damping are possible. Yet the easiest way of reducing noise is distance.
Shooting ranges are often located far from residential areas where the noise pollution
does not influence living conditions. However, shooting ranges near cities might not
have the option for distant location and other solutions for sound suppression are
required. [2, 5]
Human hearing is considered to range from 20 Hz to 20 kHz frequencies, but the
ear is more tuned to certain frequencies. The human hearing is most sensitive around the
1000 Hz frequency while sounds below 100 Hz require more intensity to be heard. The
way human hearing responds to different frequencies is seen in Figure 1. The figure
shows how the low frequency sounds require vastly stronger sound pressure levels to be
perceived by the human ear, which means that the low frequency sound requires more
energy to be audible. [6]
3
Figure 1 The human hearing responds differently to sounds depending on the frequency range and sound
pressure level (SPL). [7]
This has an influence on how the human ear interprets a firearm discharge. Sounds
around the 1000 Hz will sound more intense and therefore the greatest suppression is
needed near the 1000 Hz range. Figure 2 shows an idealized shot impulse and its
Fourier spectrum. The impulse is idealized as in real situations the surrounding area
creates reflections that affect the recording of the impulse. The peak energy in firearm
noise varies between 1000 and 2000 Hz frequencies, just around the frequencies where
the human hearing is the most sensitive. [7, 8]
Figure 2 An ideal shot impulse and its Fourier spectrum. The p+ indicates peak pressure (Pa), ta is the Aduration (ms) and f+ the frequency of the highest amplitude. [8]
4
2.1.
Reasons for sound suppressors
The muzzle report is the single loudest sound source associated with firing a hand
weapon. Depending on caliber, ammunition, environment, air pressure, temperature,
used weapon and numerous other factors, the sound levels generated by the muzzle
report usually exceed or brush close to safety limits. As an example the sound signature
of a 7.62 caliber assault rifle can be lowered from 160 dB to 140dB and below which is
considered as a risk limit in Europe. [2]
Earmuffs can turn out cumbersome and earplugs may distort sound distractingly
during hunting. Sound suppressors provide an alternative way to protect the hearing of
the hunter and those around him. The military service requires that trainers have
constant communication with conscripts during exercises, especially when firearms are
involved.
2.1.1.
Health issues
The European Union has stated 140 dB as the peak noise level allowed with impulsive
noise. An impulsive sound louder than 140 dB is considered to cause damage to
hearing. When the duration of the noise increases, a smaller sound level is required to
cause permanent damage. This comes to play in indoor shooting ranges or ranges with
shooting shelters. The echo of the muzzle blast arrives so fast that the sound impulse
seems to lengthen in duration and becomes more dangerous to hearing. [2, 6, 9]
Damage caused to hearing organs lowers the hearing threshold of the ear
permanently. This manifests as a loss of hearing and affects the whole life of the person.
The loss of hearing is most evident with older shooters who do not wear proper ear
protection and military personnel who are subject to loud noises for extended durations
during their workdays. Even sound levels of 85-90 dB have been found to induce
hearing loss if a person is exposed for long periods of time to such levels. Workers who
are exposed to sound levels over 85 dB in their working environment are entitled to
hearing protectors. [9, 10]
Extreme sounds produce pain, disorientation and eventually even death. A
deadly sound level is considered to be around 220 dB. The logarithmic scale used in the
decibel scale means that the sound level of 220 dB corresponds to about 2 MPa of
pressure. [3, 11]
2.1.2.
Hunting
Hunting situations often require that the hunter has full use of all of his senses. Earmuffs
and earplugs may hinder the hearing too much and even active hearing protectors may
distort the noises enough so that some hunters rather leave them in the car. Further
annoyance may follow as large muffs prevent fast aiming in quick situations, as they
collide with the stock of the weapon. Therefore unfortunately a large number of hunters
5
risk their hearing. In cases like these, suppressors would offer a considerable protection
both to the hunters themselves and to any animals taking part in the hunting. [12]
A notable effect provided also by a suppressor is the damping of the recoil of a
weapon. Especially younger and less experienced shooters may flinch in the anticipation
of the recoil and miss a shot because of this. A suppressor has a similar effect as a
muzzle brake. A silencer doesn’t direct the propellant gases sideways or backwards
towards the shooter, but forces a more controlled gas release that leads to smaller recoil.
Muzzle brakes also increase the noise exposure of the shooter conversely to a
suppressor. Also the flash of the gun is suppressed which may help in dim light hunting
environments. Additionally there have been reports that suppressors confuse the prey to
some degree and allow more careful placement of shots. This results in fewer wounded
animals and more controlled hunting situations. [7, 13, 14, 15]
2.1.3.
Military service
Hearing damage is a common problem during military service both with the conscripts
and the instructors. Each year the central military hospital was reported to treat around
100 conscripts with hearing problems and Finnish Health Department reports about
200-300 acute hearing traumas yearly. Heavy machinery, explosions, heavy weapons
and hand weapon fire are all possible sources for hearing damage. [10, 16]
During training earmuffs place limits on communication between the trainer and
conscripts. Orders need to be issued with sufficient volume so that everyone registers
the commands. This becomes even more important with firearms involved in the
training. Suppressors might provide an alternative for earmuffs and provide a better
communication during training. [9]
2.2.
Noise sources
Discharging a hand gun produces noise from numerous sources. The most distinct is of
course the expanding cloud of propellant gases, known as a muzzle report, which often
masks other sounds. The bullet travelling above the speed of sound produces a distinct
noise called the ballistic crack which is considered as unavoidable with supersonic
projectiles. The cycling operation of the gun is surprisingly loud but harder to hear with
the dominant muzzle report occurring at the same time. Lastly the impact of the bullet
produces its own distinct sound, which can be heard even without a suppressor as the
muzzle report has time to pass before the impact noise reaches the observer. Some
suppressors also produce an additional sound known as the first round pop. When
dealing with suppressors, the muzzle report is the main concern as a suppressor has only
minimal effect on the other sound sources. [7]
6
2.2.1.
Muzzle report
Muzzle report is caused by the expanding propellant gases combined with the air
propelled from the barrel by the accelerating bullet. These gas shock waves form a
unified shock front that is observed as the muzzle report associated with the weapons
sound signature. [4]
The damping effect of a suppressor influences mainly just the muzzle report.
The coherence of the expanding gas cloud is broken by the suppressor structure and the
energy of the propellant gas is spread out to a longer time interval, thus causing a drop
in the peak sound pressure. Oddly enough, while most of the pressure release is divided
to a longer stretch of time, the peak sound level is actually shorter in duration. This is
why there are several devices to measure the unsuppressed gunshot sound pressure
level, but only a handful of facilities that can reliably measure the sound pressure levels
of suppressed gunshots as the short peak level require exceptionally rapid response from
the measurement devices. [7]
The domination of the muzzle blast is evident when comparing Figure 3 and
Figure 4. Both plots are recorded in the front of the shooter so that the bullet flight
sound reaches the microphone before the muzzle blast. First figure shows the gunshot
from an assault rifle without a suppressor. The muzzle blast is clearly more dominating
and people rarely notice bullet flight sound when firing unsuppressed weapons. In
Figure 4 the ballistic crack has not changed in intensity but the suppressed muzzle
report is now less dominating than the ballistic crack. This also gives a limit to the
engineering of weapon suppressors. With a good suppressor the ballistic crack is louder
than the muzzle blast, and suppressing the muzzle report further becomes pointless. The
only way a suppressor could affect the ballistic crack is by lowering the speed of the
projectile, but this poses more problems, such as a limited variety of cartridges that
would function properly with the suppressor and limited accuracy in some cases. [7]
7
Figure 3 A pressure versus time plot of an unsuppressed gunshot. [7]
Figure 4 A pressure versus time plot of a suppressed gunshot. Notice the difference in y-axis scale when
compared with Figure 3. [7]
8
2.2.2.
Ballistic crack
An object travelling faster than the speed of sound produces a shock front which
expands behind the flying object and is observed as a sharp noise. This is known as the
ballistic crack. The flight sound of a bullet travelling with supersonic speed can in itself
exceed the European risk level of 140 dB when reflected from objects and surfaces back
toward the shooter. This results in a compelling argument for the use of other hearing
protectors alongside suppressors when firing in spaces with large and uniform reflecting
surfaces. Even without large echo surfaces the ballistic crack may represent a hearing
hazard as repetitive impulsive sounds exceeding levels of 110 dB have been reported to
cause hearing loss. [4, 7, 11]
The relation between bullet speed and the sound level produced by the ballistic
crack in air is seen in Figure 5. Increase in bullet flight speed is observed as a louder
ballistic flight noise. Near the speed of sound the flight sound increases rapidly and
levels out after around 400 m/s. The difference in sound pressure levels between
subsonic and supersonic projectiles is significant. Projectile shape has only a minimal
influence on the flight sound. The results were obtained by using the same weapon and
projectile and by increasing the propellant load gradually. [7]
Figure 5 The flight noise of bullets increases dramatically when the bullet overcomes the speed of sound
in air. [7]
2.2.3.
Other sound sources
The action of a firearm produces more sound than what is normally thought. The louder
muzzle report and ballistic crack mask other sounds which are a part of the weapon
cycling when fired. This is easily observed by loading a rifle with a used cartridge and
pulling the trigger. The hammer fall of an M14 rifle can exceed 108 dB within 2 meters
from the weapon. [7]
9
Sometimes when firing several shots some suppressors perform worse on the
first shot than on the following discharges. The first discharge is noticeably louder and
is known as the first round pop. When the weapon is discharged first time, the
suppressor is filled with oxygen rich air. This, combined with the pressure and heat
within the suppressor, may cause the unburned propellant, which is a part of the
expanding gases, to ignite inside the suppressor. With the second and third shot the
suppressor is filled with propellant gases and the oxygen content in the gas is not
sufficient to ignite the unburned propellant in the following shots. [13]
A noticeable sound involved also in discharging a firearm is the bullet hit sound
that is largely dependent on the target. Hitting metal targets, breaking glass and even
bullet striking flesh are reported to produce loud and distinctive sounds which may play
a major role if the aim is to hide the fact that a shot has been fired during covert military
operations. [7]
2.3.
Elimination of noise
Work safety protocols demand that workers are provided with hearing protectors when
the work place exposes them to loud noises. Hearing damage is not the only health
hazard connected to loud noises. Sleeping disturbances, hypertension and heart diseases
have all been linked to excessive noise exposure. Therefore the noise elimination has
become increasingly important in engineering and everyday life. [5, 17]
Protection from noise can be achieved with several different ways when firearms
are considered. The more traditional way to protect oneself from impulsive noise from
firearms has been ear protection. Earmuffs, earplugs and active ear protectors are a
common sight in firing ranges. Hearing protectors provide a good protection but the
protection is limited to only a single person. When the aim is to provide protection for
the surrounding environment, structural design and noise absorbing constructions must
be considered. Noise walls, sound absorptive lining in firing shelters and foliage all
provide noise absorption and protect the surroundings of firing ranges. Similar
constructions are the noise walls near motorways which protect housing from the
constant noise of traffic. [2, 7, 9]
Where ear protection provides aid for the shooter and people nearby and noise
absorbing constructions protect the surroundings, suppressors provide these both.
Limiting the sound exposure of a firearm produces benefits both to the shooter and the
surrounding environment. Limiting a shooting range only to weapons with suppressors
would provide a noticeably smaller safety area around the range as well as savings from
noise absorbing structures. [2]
2.3.1.
Protective equipment
Consumers have a wide range of personal protective equipment available to shield their
hearing from excessive noise. Earmuffs and different kinds of earplugs ensure that
everyone can find the best option for him- or herself. Some research has even been
10
carried out for producing a noise helmet to protect against low frequency noise for
military personnel and airfield operators who are exposed to intense low frequency
noise. [7, 9]
Earmuffs are an effective protection against noise but they have their own
problems. Concerning the good sides, earmuffs and earplugs suppress noise from all
sources, not just the muzzle report as a suppressor does. They provide a good level of
protection and are available in reasonable prices. Additionally with active earmuffs the
loud noises can be cut off with only minimal loss of other sounds so communication
problems related to standard ear muffs are avoidable. The drawbacks of earmuffs are
that they may hinder the shooters ability to quickly acquire a good aim as large muffs
collide with the stock of the weapon. Another drawback is that hearing protectors limit
the hunter’s ability to detect game by hearing. Even active earmuffs may distort the
sounds of approaching game so that locating the source becomes hard. Another
noticeable fact is that earmuffs don’t provide any protection to the surroundings. Other
hunters and any dogs involved in the hunt are still subject to the full noise of the
discharge. [9]
2.3.2.
Protection of the environment
When considering shooting ranges the noise pollution can be lowered with several
ways. The shooting range can of course be located in remote areas to protect humans
from noise and sometimes enclosing the range indoors can be used as well. Both are
options when a new range is being founded but when improving existing ranges the
options are fewer. Three main ways available at this point are using protective
equipment, building noise walls and structures to capture the noise and finally the use of
suppressors. [2]
It’s considered that 65 dB is a reasonable sound level limit allowed in houses
near shooting ranges. This means that the shooting range has either to spend
considerable amount of money to build sound barriers or that a sufficiently large area
around the range is reserved for the attenuation of shots. Another option is the use of
subsonic loads and sound suppressors. With shotguns the use of suppressors and
subsonic cartridges produces a suppression of -12 dB which doesn’t compare well with
rifle suppressors, but still manages to decrease the required space of a shooting range to
1/16th of the original as seen in Figure 6. This opens up a number of possibilities for
new shooting ranges in more heavily populated areas. [2]
11
Figure 6 The space required around a shotgun shooting range without sound suppression from
suppressors and subsonic loads and with the suppression methods in use. [7]
2.3.3.
Noise damping structures in shooting ranges
Shooting ranges have different kinds of structures which influence the behavior of
sound waves. Structures intended to protect the surrounding environment from shooting
noise consist of noise walls, shelters, foliage, and other possible noise barriers. Noise
walls usually surround the back of the range and act as safety measures and as noise
control. The attenuation provided by a noise wall ranges from 5 to 10 dB near the range
and from 0 to 7 dB within a kilometer from the shooting range. Within 20 meters from
the shooter a shooting shelter provides attenuation of 3-15 dB, depending on the
shooting direction and an attenuation of 0-4 dB within a kilometer. The effect of foliage
on shooting noise is less consistent and rarely seen as an active attempt to provide
attenuation. [6]
Structures intended to protect the shooter and possible trainers are seldom used
in outdoor ranges and they mostly consist of absorptive surface materials used in
shooting shelter walls and roofs. Figure 7 shows a possible structure which could be
utilized in shooting ranges to enhance noise attenuation. [7]
12
Figure 7 Noise barriers for effective elimination of sound leaving the shooting range. [7]
Shooting shelters provide protection from wind and rain as well as attenuate
noise from the weapon fire. With unsuppressed weapons the shooting shelter extends
the time for which the shooter and observers are exposed to peak sound pressures.
Sound reflected from hard wooden or concrete surfaces increases the risk of hearing
damage. Therefore ranges intended for unsuppressed weapons should take advantage of
absorptive materials in the shelter roof and walls. This provides protection to both the
participants and surrounding environment. Suppressed weapons benefit less from the
attenuation of absorptive shelter materials. The peak sound pressure is lower and shorter
in duration and main sound exposure results from the ballistic crack. Figure 8 illustrates
the difference. [7]
13
Figure 8 To reduce noise pollution the use of thick and absorptive materials is encouraged with
unsuppressed weapons. With suppressed weapons an open structure with a roof is sufficient. [7]
2.3.4.
Industrial sound suppressors
Suppressors found in industrial use like air ducts and car exhaust noise control share
common features with firearm suppressors. Two main factors in industrial sound
attenuation are absorbing materials and wave guidance. Absorptive materials are often
porous with open cells, fibrous and possibly soft materials. Sound waves attenuate in
such materials as pressure variations and gas viscosity turn coherent waves into
turbulent flow and heat. Porous foam panels in concert halls are the clearest examples of
the material. [5]
Wave guidance is the second major method for attenuation and more often seen
with car mufflers and air ducts. Sound in wave guides like pipes and air ducts travels
easily far, which is why almost every single air ventilation system has some kind of
silencer or suppressor. Easiest way to provide attenuation is a sudden change in the duct
cross section and in the following expansion chamber. This discontinuation divests
energy form the sound wave and reflects part of it towards the source. Reflected sound
waves interact and distort sound waves coming from the source and provide attenuation.
[5]
14
A common acoustical structure found in acoustic design is the Helmholtz
resonator. Helmholtz resonator is a structure where a larger chamber is connected to the
rest of the system by a neck with smaller diameter. This kind of structure works as an
acoustic spring and it can be used to identify sound tones and eliminate them. The most
common example of a Helmholtz resonator is an open bottle when blowing gently
across the mouth of a glass bottle. [5, 18]
The ideas behind expansion chambers and Helmholtz resonators can be seen in
Figure 9. Car mufflers utilize expansion chambers to let sound waves reflect and cancel
each other. The Helmholtz resonator is used in the muffler’s resonator chamber which is
tuned to attenuate certain frequencies and get the best effect on the most important part
of the noise spectrum. [19]
Figure 9 A cut open view of a car muffler. [19]
2.3.5.
Suppressor structure elements
Firearm suppressors rely partly on same principles as other industrial suppressors.
Diverting gas flow with different channels and prohibiting free flow of gas with baffle
structures are among the most used methods. Common structure elements in
suppressors are also expansion chambers, wire mesh, stepped or threaded surfaces and
reflection discs. [7]
A baffle is in its simplest form a disc with a hole in the center for the bullet to
pass through. The bullet seals the chamber behind it for a moment as it passes through
the hole in baffle. Propellant gases following or overtaking the bullet are reflected and
diverted. More advanced versions of baffles are conical in shape to further enhance the
diversion of gases and may contain channels or grooves to utilize the gas flow against
the expanding gas cloud. For example a gas stream may be diverted across the bullet
path right behind the bullet to push the following stream into an expansion chamber. [7]
Expansion chambers are empty spaces inside the suppressor which provide the
expanding gas cloud with space to consume its energy via expansion, heat transfer and
turbulence. Especially reflex designs rely on large primary expansion chambers as the
15
sleeve extending over the barrel allows free space. Expansion chambers also allow
sound waves to reflect back towards the source for added attenuation and turbulence. [7]
Surface area increases heat diffusion between expanding gas and surrounding
suppressor structure. Cooling down the propellant gas is one way to rob energy from the
expanding gas and to provide attenuation. Wire mesh is a good element when surface
area is needed and a wire mesh filling in expansion chambers is an element which has
been seen since the earliest suppressor models. The same idea is behind fully threaded
outer shells and stepped cone baffles where otherwise normal slanted wall is replaced
with surface modification which increases the surface area available for contact with the
gas. Additional effect found with modified surfaces is increase in turbulence.
Turbulence forms circular flows within the expanding gas and plays a major role in
sound attenuation. [3, 5, 7]
Some suppressors are designed to utilize additional medium for optimal
performance. These are mostly seen with pistol caliber suppressors and are known as
“wet cans”. The suppressor is filled with different kinds of liquid or foam substances
which increase the heat transfer and sound attenuation. Wet suppressors can be designed
to even smaller dimensions but their performance is less optimal if used dry. A negative
aspect is that the substance might not always be available and also during discharge part
of the liquid substance may end up on the shooter. Any hazardous liquids might pose a
health hazard for the operator. [7, 20]
16
THE AIM OF THE WORK
The aim of the research project was to improve an existing suppressor design by
replacing most if not all of the existing suppressor materials with lightweight
alternatives which could still operate under the demanding conditions a suppressor
experiences.
The search for alternative materials was conducted with the aid of a computer
software designed for material selection. The main goal of the software was to provide
multiple alternative materials from which the best candidates could be chosen for use in
suppressor components. The final aim of the study was to test these components by
building a prototype suppressor to be tested in live fire conditions to determine their
final suitability.
17
3.
SUPPRESSOR PROTOTYPES
The design of a new suppressor prototype was started on top of an already existing
suppressor. The previous model was a reflex type steel suppressor with a robust design.
The new prototype was aiming for lighter materials and structural solutions which
would work with several materials. Particular interest was addressed towards carbon
fiber composites and aluminum alloys for their light weight, mechanical properties and
availability.
The design process started on the outer shell of the initial structure and worked
from there inwards. New materials raised questions especially on joining the parts made
from different materials together so that the whole suppressor would withstand the load.
For example the high thermal expansion of aluminum might cause stresses and failure
when combined with other materials as the temperature variations may be significant
within the suppressor.
3.1.
Structures and designs
First patents related to suppressors are dated to 1899. J. Borrensen and S. Sigbjornsen
filed patents on expansion chambers and baffles, the same ideas which are still found in
modern suppressors. Although the first patents were filed in 1899, even older attempts
and designs of suppressors have been recorded. The first commercially successful
suppressor was the Maxim Model 1908, developed by the Hiram Percey Maxim. [7]
Suppressors can be divided roughly into three groups by their structure. More
traditional suppressors are attached to the end of the barrel and they extend outwards.
This kind of suppressor usually consists of expansion chambers, baffles and in some
cases wire mesh. There are several good sides in the design. For example, the
suppressor is compatible with several different weapons with only minimal work as the
barrel rarely limits the use. In some cases the barrel might even be shortened to retain
the original handling characteristics of the weapon. The suppressor is usually easy to
attach and several quick lock mechanisms are available for suppressors in this variety.
The Maxim model 1910, one of the first commercial suppressors ever, in Figure 10 and
the more modern Vaimeco .30 SX in Figure 11 are good examples of this category. The
Maxim suppressor has a closed structure and any maintenance to the suppressor is very
limited. The Vaimeco .30 suppressor is made of anodized aluminum which provides a
lightweight structure with only a few detachable parts which enables easy maintenance.
[7]
18
Figure 10 A Maxim Model 1910 .44 suppressor for center fire weapons. [7]
Figure 11 Vaimeco .30 SX suppressor with a limited number of parts to enable easy maintenance. [7]
Another traditional suppressor, the Hush Puppy used by U.S. Navy, uses
polymer wipes for extra attenuation efficiency. The wipes are seen in Figure 12 as parts
with cross shaped cut in the surface. The wipes were efficient in providing suppression
and slowing down the bullet to subsonic velocities. The drawbacks were the highly
reduced accuracy and the limited lifetime of the polymer wipes. Polymer wipes have
19
almost disappeared from U.S. civilian markets due to the problematic legislation which
makes replaceable parts within suppressors extremely expensive. Disappearance from
civilian markets also severely hinders research on the subject. Other countries with less
limiting legislation have still continued the research to some extent. [7]
Figure 12 A second generation clone of the “Hush Puppy” suppressor. [7]
The second group is known as reflex suppressors. The idea is that the suppressor
extends back over the barrel of the weapon like a sleeve instead of protruding forwards.
This enables the use of massive primary expansion chambers and the shorter barrel
length is easier to use and operate. Reflex designs also offer more durable and robust
structures, as less stress is placed on the barrel due to the two attachment points for the
suppressor. One point of attachment is usually utilizing threads at the end of the barrel
similarly to traditional designs and the second attachment point is usually located at the
back of the suppressor where it surrounds the weapon barrel. The BR-Tuote .50
suppressor seen in Figure 13 is a good example of the reflex design. It consists of a
large main expansion chamber and a small coaxial insert that plays the role of baffles.
The insert doesn’t provide the best possible sound reduction, but the main purpose of a
suppressor on a .50 caliber rifle is to reduce the muzzle blast and recoil and to hide the
muzzle flash instead of silence. The suppressed sound level is still around 161 dB. [3]
20
Figure 13 .50 BMG caliber from BR-tuote consists mainly of a large expansion chamber and small
baffles. [3]
Another example of the robust reflex design is the BR-tuote TX8 seen in Figure
14. The steel structure offers durability and strength even under high stress situations
like rapid fire. The construction is not meant for disassembly and any cleaning has to be
done by submerging the suppressor in solvents. On the other hand, the steel structure
requires very little maintenance. The TX-8 relies on large primary expansion chamber
and on a limited number of baffles. The primary blast baffle is provided with a
deflection cone that further disrupts the forward flow of expanding gases. [3, 21]
Figure 14 A cut out of the Br-Tuote TX-8 reflex rifle suppressor attached onto a gun barrel. [21]
The third suppressor group is known as integral suppressors. Integral
suppressors are built into the firearm and require a lot more modifications to the weapon
than the previous two groups. Integral suppressor design takes the idea of reflex
suppressor and develops it further. The suppressor envelopes the entire barrel and
becomes an integral part of the weapon. A weapon with integral silencer design cannot
be used unsuppressed without further serious modifications. An integral silencer acts as
a huge expansion chamber around the barrel. Several ports are often installed into the
barrel itself so that expanding propellant gas could be diverted from the barrel. The
bullet speed decreases significantly, all the way to subsonic velocities if needed. Figure
15 shows the integrally suppressed rifle AWC R 10 to illustrate the design. The
expansion chamber around the barrel is divided into two different chambers by a spacer
in the barrel. The ventilation on the barrel reduces the pressure behind the projectile
slowing it down to subsonic velocities and eliminates the ballistic crack. The large
expansion chamber design also attenuates the muzzle report. The decrease in bullet
21
velocity limits the number of usable cartridges for the weapon. This suppressor design
could provide a net sound reduction of 28 dB. [7]
Figure 15 The integrally suppressed rifle AWC R10 with modified barrel and stock to house the silencer
sleeve. The modified barrel is at the bottom of the picture and the suppressor sleeve above it. [7]
3.1.1.
Initial structure
The design process started from an already existing steel-based design. The initial
design was a reflex type suppressor with one main expansion chamber and seven
smaller chambers separated with steel baffles. The baffles are fixed with welded support
rods to the part attaching the suppressor to the weapons barrel. The design is sturdy and
robust, but the maintenance possibilities are limited. The main disadvantage with the
initial design was the weight. A steel suppressor weighing between 0.5 – 1 kg is a
noticeable increase in the weight of the weapon, especially if it has to be carried over
longer distances.
3.1.2.
First prototype
The first prototype was designed primarily to test the carbon fiber strengthened polymer
matrix composite as the outer shell material. The internal components were almost
identical to the initial structure with only some exceptions to allow the attachment of the
outer shell without altering the carbon fiber tube. The outer shell was a carbon fiber
composite tube which was attached to the internal structure by compressing it between
end caps. The first prototype can be seen as disassembled in Figure 16.
22
Figure 16 The first prototype as disassembled. The internal structure was mostly
unchanged as compared to the initial design, because the testing was focused on the
carbon fiber outer shell. At the bottom is the internal steel structure and above it the
carbon fiber outer shell and steel end cap.
3.1.3.
The second prototype
The second prototype consists of a carbon fiber outer shell with metallic inserts at both
ends and of an anodized aluminum internal structure. Metallic inserts in the outer shell
enable a wider variety of internal designs as there is no need to apply pressure to the
outer shell from both ends for the suppressor to remain compact. The threaded metallic
inserts are visible in Figure 17.
23
Figure 17 Metallic inserts of the carbon fiber composite outer shell in the second prototype.
Although the outer shell of the second prototype was finished when this was written, the
internal structure was still under design. A design draft of the second prototype is seen
in Figure 18. The aim of the design is to allow the disassembly without specialized
tools for easy maintenance and the minimization of welding. The baffles of the design
are single modules and replaceable to compensate the likely quicker deterioration of
aluminum parts when compared with the original steel structure. The biggest challenges
considering this design are how well the aluminum can replace the steel and the
eventual problems with manufacturing the structure.
24
Figure 18 A design draft of the second prototype.
3.1.4.
Future prototypes
The testing of the second prototype will show where the future development should be
directed. If the structure of the second prototype turns out to be durable enough to work
with rifle caliber weapons, the next logical aim would be to increase the sound damping
properties. This could be achieved by modifying the shape and number of baffles and by
increasing the size of the primary expansion chamber. Diverting a bigger part of the
propulsion gas towards the back of the suppressor could also add to the damping effect.
3.2.
Materials
The materials of suppressors play central role in the construction. Wrong material
selection might result in poor performance, reduced service time or even in failure with
serious consequences. Sudden changes in external conditions during discharge set a
variety of requirements to the materials. Even when a material might otherwise be
suitable for use in suppressors, the compatibility with other materials and fabrication
problems may limit its usability. All these things have to be taken into consideration
during the design process.
3.2.1.
Material requirements
When a rifle caliber weapon is discharged the conditions inside and around the barrel
and muzzle change rapidly. At 200 microseconds the bullet has already left the barrel
and the propellant gases expand and accelerate to high speed. The temperature of the
propellant gases flowing from the barrel can achieve levels as high as 1300 °K. The
pressure of the expanding gas cloud can be as high as 60 MPa and the speed of the
flowing gas and possible debris like unburned propellant is reported to be around 1000-
25
1400 m/s. It is clear that conditions like these set a wide range of requirements on the
material which needs to endure in suppressor applications. [4]
The weapons used in hunting and outdoor shooting ranges are also exposed to
rain, ultraviolet light, wear, collisions and other related conditions. The chemical
composition of gunpowder residue and propellant gas might turn out to be hazardous for
the materials.
3.2.2.
Materials used in suppressors
The materials used in suppressors are dominantly metals. Easy access, reasonable price,
wide variety of fabrication processes and durability are properties which have played a
major role in the selection. The most common metals are stainless steels in suppressors
for weapons using centerfire cartridges and aluminum in suppressors for rimfire
cartridge weapons where the gas pressure is lower. The ease of fabrication and low
weight of aluminum make it an excellent material in places where its mechanical
properties are sufficient. With larger calibers, aluminum lacks abrasive wear resistance
and high temperature strength to withstand prolonged use. [3]
Tests have been performed on the use of ceramic materials in baffles to utilize
the abrasion and heat resistance of the material. Unfortunately the fragility and low
thermal shock resistance have limited the use of ceramics. Rapid and intensive
temperature variations and impulsive pressure shocks limit the use of more exotic
materials like plastics. Therefore material research has focused into more specialized
metals. Titanium has been found to work in suppressors and the only limiting factors for
its wider use have been the price of the material and fabrication problems. [3]
Steel
Even though different materials are becoming more common in rifle caliber
suppressors, steel still holds the dominant position. Easy fabrication, wide variety of
different alloys and high strength properties still weigh much in the selection. For
smaller manufacturers the low price of the material and wide availability may also play
a great role. Drawbacks of the material are its density and weight. A suppressor made
out of steel may weigh around 0.5-1 kg which is not much if the weapon is used for
practicing shooting but a hunter rarely wants to carry any extra weight.
When used with sport shooting or during practicing a steel suppressor heats up
rather fast. Even though the heat might not damage the suppressor it may still produce
problems. A hot object produces optical distortions as the air above it heats up. This in
turn may cause problems in aiming as the aim line is located right above the suppressor.
Steels have a wide variety of alloys and modifications available for use. Most of the
steels used in suppressors are of stainless type. Corrosion resistance provided by the
chromium is useful if the weapon is used under wet conditions. [7]
26
Aluminium
Aluminum is used in suppressors for smaller caliber weapons. Aluminum itself is soft
and has limited mechanical properties as compared to steels. In suppressors the
materials are subject to thermal stresses, violent pressure variations and abrasive wear
from unburned propellant particles and this shortens the life span of aluminum parts.
Aluminum parts are usually anodized to grow a thick protective oxide layer on the
surface. Aluminum may also create problems when used in combination with other
materials as the thermal expansion of aluminum tends to be greater than that of many
other materials used in silencers. This may create a problem when aluminum parts are
tightly fitted inside other materials and the materials expand at different rates during
heating. [3]
The benefits of aluminum include its good availability, light weight, cost
efficiency and many manufacturing possibilities. Light weight becomes more important
than extensive heat resistance or excellent mechanical properties, when the weapon is
used, for instance, in hunting. Hunting situations rarely require extensive numbers of
shots to be fired in close succession. Smaller thermal stresses enable the use of
aluminum parts and provide a more pleasant hunting experience with less weight
encumbering the weapon during travel and aiming.
Carbon fiber composite
Carbon fiber composites consist of a polymer matrix and carbon fiber particles or fibers
for reinforcement. The material is strong of light weight and it is often seen in aerospace
applications and in high end sports equipment. Anything from tennis rackets to airplane
fuselages are made out of carbon fiber composites. [22]
Carbon fiber is used in suppressors mainly with pistol caliber weapons. This is
because the polymer matrix in carbon fiber composites is susceptible against the
combination of heat and mechanical stress and the peak temperatures and pressures stay
lower with pistol caliber weapons. Continuous fire will cause melting of the matrix
material and extensive deterioration of the matrix may result in serious damage to both
the suppressor and the weapon. The benefits of carbon fiber composites come from the
light weight of the material combined with good mechanical properties. [7]
The manufacturing process of carbon fiber composite materials and components
introduces to the design of parts some limits, which the traditional metallic components
can ignore. For instance, any shaving or grinding which damages the carbon fibers may
result in a significant decrease in mechanical properties. This means that parts made out
of carbon fiber composites must be manufactured as close to the final shape as possible
to minimize the damage to the fibers during shaping. [22]
27
Titanium
Titanium is a light weight metal with a multitude of uses. Titanium and its alloys can be
found in aerospace, chemical industry, high end sports equipment and even in medical
applications. It is not uncommon for the head of a golf club or for a medical implant
used in bone surgery to be made from titanium alloys. Good mechanical properties and
excellent chemical properties combined with the light weight are the key features of the
material. Titanium forms a passive oxide layer on the surface which makes the material
inert under many demanding corrosive conditions. The drawback is the challenging
manufacturing process. Especially hot titanium is highly reactive with the environment.
Therefore titanium can only be melted either in inert atmosphere or in vacuum. The use
of titanium in suppressors is gaining popularity, but manufacturing costs still keep
titanium suppressors as a high end solution. [3, 23, 24]
Other materials
There have been studies on other possible materials for suppressor construction.
Research on ceramic materials showed that more common ceramics tend to be too
brittle to withstand the heat and pressure shocks and the abrasive effect of unburned
powder, especially under rapid fire. Polymer materials have been used mostly as rubber
wipes which the bullet punches through and as the matrix of composite materials. The
rubber wipe technology had adverse effect on bullet velocity and accuracy but provided
excellent damping effect. The drawback was that the wipes wore down in 10-20 shots.
Composite materials have been mostly limited to carbon fiber composite outer casings
in pistol caliber suppressors. [3]
3.2.3.
Search for alternative materials
The search for new alternative materials for suppressors requires either clear knowledge
on the conditions the materials need to withstand or knowledge on several materials that
are already used in the application. With exact requirements the search for possible
materials for the application becomes easier. If the requirements are not definite and
there is available only a list of materials already found, the process becomes more
challenging, as the requirements are formed by finding properties common to the known
materials and by defining which of these properties are critical for the application.
The criteria used for materials in suppressor applications are dependent on what
kind of firearm the suppressor is intended for. A .22 caliber pistol suppressor doesn’t
require as high strength and hardness as a .308 caliber suppressor for a hunting rifle due
to the fact that the gunpowder load and pressure of the propellant gases are smaller.
Even higher requirements are placed on the heat and abrasion resistance of the material
used in a 7.62 assault rifle suppressor which is subjected to rapid fire. With lower
requirements comes a broader pool of materials to choose from.
28
The suppressor on which this study concentrates is meant for hunting and target
practice weapons of possibly .308 caliber. This sets definite strength requirements on
the material due to the high pressures involved. Hunting weapons rarely require high
service temperature from the suppressor material as the number of shots fired in a
hunting situation rarely exceeds three to four shots. Zeroing the weapon or practice
shooting might cause some heat accumulation, but long intervals between shots also
reduce the effect of heat shock which is dangerous for many brittle materials like
ceramics.
Material selection software
New materials were searched for with the Cambridge Engineering Selector (CES)
software. The software compares given requirements to a database of materials and
rules out materials which cannot fulfill the set requirements. The selection process was
twofold. First part of the process involved finding a set of requirements which accepted
materials which are known to work with suppressors and ruled out materials which
would not withstand the preset circumstances. The second part was to explore the
material groups the software recommended for the application and to see which of them
were viable and interesting for further research.
The CES software offers two options for filtering and selection of materials.
Filtering based on values and filtering based on graphical selection maps. The value
based limits can be used to define such characteristics as resistance to oxidation in sea
water or resistance to ultraviolet radiation. Graphical material selection maps are more
versatile tools where the user defines one or two material properties that act as axes of
the selection maps. The software then plots different material groups on the map
according to their material properties. A larger size of a material group means more
variance on the material properties of the select material group. The user is then able to
define certain areas or parts of the selection map that fulfill the needed requirements.
In this study the desired materials were selected by first using a value based
filter to define some ground requirements the materials should pass and after that the
remaining material pool was further defined with a series of material selection maps.
Each selection map removed a number of materials from the pool of available materials
which could not fulfill the needed requirements. The most common way to use the
selection maps in this study was to plot a map of two defining characteristics and then
choose two materials which were known to function in suppressors and draw a limiting
line between these two materials. Materials which fell on the wrong side of the selection
line were eliminated from the pool of available materials. To clarify the selection
process, the areas which were excluded from material pool are colored light red on the
selection maps.
29
Chemical and thermal requirements
The search for new possible materials began with defining some lower limits for
environmental resistance which would not rule out any materials already found in use
with suppressors. Finally a combination which required reasonable resistance from the
materials without ruling out viable materials was found. The average resistance
requirements to flammability and corrosion resistance in fresh water and sea water were
set and less strict requirements were placed on wear resistance, oxidation at 500°C and
the resistance against organic solvents. Higher resistance to oxidation would have been
preferable, but under closer examination this did not form a bigger problem. Oxygen
content within the suppressor decreases when the weapon is discharged as the propellant
gas replaces fresh air. Therefore high temperature oxidation is less likely to occur.
The next requirement for candidate materials was the temperature which the
materials would have to withstand under normal operation. Propellant gases reaching
1000°C gave the theoretical maximum limit. This, however, was a too strict requirement
as under normal operation a hunting weapon suppressor would not face rapid fire and
the suppressor material would require very extensive use to reach the same temperature
as the propellant gases. Solution was found by finding a limit which accepted most
materials seen in suppressor applications. The limit was set to 180°C. The selected
temperature limit and maximum service temperatures of different materials are shown
in Figure 19. Known suppressor materials were highlighted to show how they relate to
each other. Steel obtained a clear advantage to the other materials when the heat
resistance was considered.
Figure 19 Selected maximum service temperature range and materials known to be used with
suppressors applications. [25]
Heat resistance was not the only thermal demand which the material needed to
fulfill. Many ceramic materials, for instance, meet the 450°K requirement but would
still not work properly under real conditions. This had been established by previous
30
researchers as the rapid temperature changes and abrasive wear managed to demolish
tungsten carbide inserts in suppressor tests with rapid fire weapons [3]. A way to
describe a materials ability to withstand rapidly changing temperatures was found by
considering the thermal shock resistance. Thermal shock resistance is best defined as
the relation between the normalized strength and the linear expansion coefficient of a
material. Materials that break easily from thermal shocks have a high linear expansion
coefficient and small normalized strength. In other words the material distorts
considerably under thermal strain but cannot withstand the strong internal stress caused
by this distortion. The internal strength is dependent on different properties with
different material groups. Metals and polymers are defined by their ultimate tensile
yield strength, ceramics by their modulus of rupture and composites by their tensile
strength. Different properties are used as the different material groups experience failure
different ways. Whereas brittle ceramics crack and composite materials tear from
delamination or matrix failure, metals and elastomers deform until failure occurs.
However, the differences in thermal shock resistance are rather small between
the materials under scrutiny for suppressor applications. Only ceramics and ice would
be excluded if thermal shock were used as a filtering factor. Instead of thermal shock a
better factor for thermal stability was found from thermal distortion. This is best
described using the material properties of thermal expansion and thermal conductivity.
High thermal expansion produces strain in the material as temperature changes, whereas
high thermal conductivity levels out the temperature variations faster through the
material. With high thermal expansion and low thermal conductivity the material
expands and contracts strongly as temperature increases, but the strain is limited to local
areas. This leads to significant internal stress states. Low thermal expansion and high
conductivity leads to smaller changes in dimensions even with large temperature
variations and the changes are spread more evenly. Consequently the internal stresses
stay at minimum. A filtering relation between these two factors was found by using
materials already found in suppressors as reference. The division between materials
which seemed plausible alternatives for their thermal distortion resistance is shown in
Figure 20. The dividing line filtered materials with high thermal expansion and low
thermal conductivity out of the material pool. The dividing line was set to include both
epoxy/glass fiber composites and polyimide material groups, but exclude material
groups with worse thermal distortion properties. The materials that were excluded are
the ones that located in the light red area. Materials known in suppressor applications
are highlighted in the picture. Material groups marked with grey have been filtered out
in previous limiting stages. [3, 26]
31
Figure 20 Thermal expansion versus thermal conductivity of materials. The highlighted materials are
known to be used with suppressors. [25]
Mechanical requirements
After the thermal properties were used for filtering, mechanical properties became a
topic of interest. Stiffness and impact resistance were the first mechanical properties
required from the materials. High stiffness was needed to ensure that the expanding
propellant gases would not distort the structure too much to block the projectile path at
any point. The blocked path would have resulted even in the best case in some degree of
damage to the suppressor structure and in a clear loss of accuracy. At worst the blocked
bullet path could have led to explosive failure of the suppressor and damage to both the
weapon and the shooter. Impact resistance ruled out materials which are too brittle to
withstand rougher circumstances experienced in hunting trips as well as the blast wave
of the propellant gases. Impact resistance is best described in this kind of application by
comparing materials fracture toughness to its Young’s modulus. High Young’s modulus
relates to stiffer materials and high fracture toughness coefficient relates to material’s
ability to withstand fracturing stress. Figure 21 shows how this relation was used to
filter out materials too susceptible to impact fractures. Known suppressor materials are
highlighted and named in the figure. The dividing line was drawn so that material
groups Polyethylene Ether Keytone and Wrought Austenitic Stainless Steel would be
included but materials with worse relationship between fracture toughness and Young’s
modulus than these two were excluded.
32
Figure 21 Fracture toughness versus Young’s modulus describing materials ability to withstand impact
loads. [25]
Stiffness of material is tied to its Young’s modulus which describes the amount of strain
produced by certain stress. Stiffer materials deform less under stress and have a higher
Young’s modulus. This in itself may not be sufficient when the application is looking
for stiffness with minimal weight. Mass sensitive high stiffness applications therefore
prefer to use the relation between Young’s modulus and material density to find optimal
materials. This relation was used in Figure 22 to filter materials that wouldn’t provide
enough stiffness or would require too much weight to produce good results. The
materials used as anchor points for dividing line were Polyimide and Wrought
Austenitic Stainless Steel.
Figure 22 Material stiffness represented by Young’s modulus versus material density. [25]
33
After applying the cumulative list of limiting filters to all materials which were
in the CES database, a pool of 72 materials remained. The alternatives could be divided
into smaller groups. Steels and cast irons, polymer matrix composites, aluminum alloys,
high temperature superalloys, titanium alloys and nickel alloys formed the largest
groups. The next filtering operation was to consider the possible costs and availability
of the materials. For example aluminum fiber within aluminum matrix may provide the
needed properties but the costs and availability ruled the material out of the pool.
The filtering process for removing the most expensive and exotic materials out
of the pool as they would not be viable options for a commercial product due to their
high price is shown in Figure 23. The limiting price was set to around 60 GBP (the
program uses pounds as the default currency) per kilogram. The price versus kilogram
comparison is not the best one to rank the materials for viability as the density and other
properties should also be considered before any ranking is reasonable.
Figure 23 Price per kilogram-filtering aims for removing too exotic alternatives out of the material pool.
[25]
After filtering out most exotic alternatives, a pool of 64 material groups
remained. The pool of interesting materials could further be reduced by removing the
material groups that were already used with suppressors as the intention was to find new
alternative materials. This took out many aluminum alloys and steels, carbon fiber
composite with traditional epoxy resin and titanium.
The remaining material groups worth of closer inspection included aluminum
bronze, aluminum/silicon carbide composite, nickel tungsten alloy, chromium nickel
alloy, cobalt superalloy, zirconium of several grades, nickel alloys, magnesium/silicon
carbide composite, molybdenum alloys and polyimide graphite composite.
34
Filtering results
The CES software provided also data sheets on material properties which
provided an opportunity to compare candidate materials with each other with more
precision. The following material descriptions and properties were provided by the
software for the materials listed above.
Aluminum bronze is a copper-aluminum alloy. It is used where high strength is
needed in combination with corrosion resistance. The alloy is found in marine and
chemical applications, pumps, valve gears, chain condensers and heat exchangers for
acids and salts. The benefits would have made the alloy a viable alternative, but the
density of the material which is only slightly smaller than that of cast austenitic steel
yielded the same problems that the original suppressor had. Weight savings would be
only minimal, if steel were to be replaced with aluminum bronze as the suppressor
material. The material could be considered for use in selected parts like the first baffle
which takes majority of the wear and strain. The formability and welding properties of
aluminum bronze would also limit its use. Aluminum bronze parts are usually cast and
the casting process of the material is demanding.
Aluminum/silicon carbide composite is a metal matrix composite which means
that the material has an extremely wide price range and a wide variety of compositions.
The material is used in electronics packaging, pistons, engine parts, mountain bike
frames and precision instruments. The ceramic silicon carbide is used to counterweigh
the high thermal expansion coefficient of aluminum and it can be used to tailor the
expansion to a specific range, if needed. Otherwise the material is stronger than
aluminum and has better mechanical properties. Unfortunately the low maximum
service temperature and limited ductility makes it unsuitable for suppressor application.
Nickel alloys are found in applications where high heat resistance and oxidation
resistance are needed. Nickel alloys like Inconel are used in furnace shields, gas
turbines, combustion liners, chemical plant hardware and also in suppressors. Inconel
blast baffles are found in some high end suppressors where steel has not been sufficient
to withstand the circumstances, like in rapid fire weapons. Density which is comparable
with steel limited its viability to anything else than single parts like primary blast baffles
for this research. [3, 27]
Cobalt superalloys are alloys used in high temperature applications with cobalt
as the main constituent element. They are found in aerospace engines, turbine blades,
afterburner parts, furnace parts and combustion chambers. Excellent chemical resistance
and mechanical properties would have made them viable for applications like
suppressors. The major drawback of the material group was the high density. When
being even heavier than steel, there would have been no point in using the material in
larger extent. Even use in smaller parts would have been unreasonable, as less dense and
cheaper alternatives were available.
Zirconium is in its pure form a transition metal, but rarely used as unalloyed.
Zirconium alloys have excellent chemical properties and are used with chemical
processing equipment, heat exchangers, pipes and pumps. Small trace particles of
35
zirconium can be a fire hazard and toxic if ingested in large quantities. Zirconium has
limited weldability and can be welded only to titanium, vanadium and niobium. For this
application the limited possibilities of obtaining and joining zirconium parts formed a
problem. Zirconium is lighter than steel but still not light enough to be considered for
major components.
Magnesium/silicon carbide composite is in some respect similar to
aluminum/silicon carbide composite. Magnesium-based composite has slightly better
mechanical properties and is lighter than the aluminum-based composite, but it has
slightly weaker thermal properties including maximum service temperature.
Magnesium/silicon carbide composite can be found in aerospace applications, pistons,
rods and rocker arms, even in special horse shoes. The material was an interesting
competitor for aluminum in major structural elements for suppressor applications even
with the high price of magnesium composites. The weaker thermal properties would
have required separate testing before anything definite for its usability could be stated.
Molybdenum alloys are extremely hard materials, used with high temperature
applications. Molybdenum is found in nuclear reactors, die casting cores, radiation
shields and in aerospace parts. Excellent mechanical and chemical properties are
hampered by the high density of the material, which is around 20-25 % higher than that
of steel. Molybdenum is considered highly toxic which also limits its use. For this study
molybdenum could have been considered only in minor parts and even there lighter
alternatives would have been available.
Polyimide carbon is a carbon fiber composite with a polymer matrix designed
for higher temperature applications. Mechanical properties of polyimide matrix and
more traditional epoxy matrix composites are somewhat similar. The difference comes
from better thermal properties. Maximum service temperature difference between the
two carbon fiber composites is at least 10% in favor of the polyimide matrix composite.
All in all the carbon fiber composite with polyimide matrix was deemed as an
interesting material for further studies.
36
4.
PROTOTYPE TESTING
Suppressor performance testing usually focuses on measuring the sound attenuation
which they provide. This measurement is performed at specific places around the
weapon during discharge and usually more than one microphone is used in order to get
a more complete picture on the propagation of the sound impulse around the weapon.
Tests are also performed with only a single microphone, but these give only a limited
description of the suppressor performance. The tests usually follow the procedures
defined in U.S. Army standard MIL-STD-1474C to produce reliable and comparable
results.
The typical placing of microphones is shown in Figure 24. The use of multiple
microphones provides a more complete picture on the performance of the suppressor.
The most important microphone placement is right next to the shooter (A), to measure
the sound pressures the shooter’s ear is subjected to. Microphones located at the back
(E) and to the side (D) of the shooter show how the surroundings are affected by the
discharge noise. Usually strongest damping is found in the direction directly behind the
shooter (E). The side microphone (D) measures how a fellow shooter in a firing range
might observe the discharge. Finally the microphones at the front of the shooter (B and
C) provide info on how the attenuation in the frontal sector is affected by the
suppressor. The frontal sector is subject to greatest sound pressure levels during
discharge and suppressors rarely can influence these levels to any great extent. This is a
result of the bullet flight noise becoming a dominant sound source. [7, 28]
37
Figure 24 A schematic presentation of the usual placement of microphones during the measurement of
suppressor performance. [29]
Sound pressure level measurement is often connected to occupational safety,
where the way by which the human hearing reacts to different sounds plays a great role.
The different audible range of varying frequencies has led to the development of
different weighing systems seen in Figure 25. The weighing systems are basically a set
of filters which modify the actual sound impulse by both amplifying some frequencies
and suppressing other frequencies. The weighing systems are used to simulate how the
human hearing interprets the noise of different frequencies under different
circumstances and they are used in sound measuring implements.
38
Figure 25 Different weighing systems used with sound level measurement. [7]
The human hearing is most sensitive around the 1000 to 2000 Hz frequency range
which is evident in Figure 1. Therefore any noise outside that range would require more
sound pressure for the human ear to react to it. The A-weighing simulates this by
damping any noise outside that range. As the A-weighing is best interpreting the human
hearing it is the most commonly used system with occupational safety measurements.
C-weighing provides less attenuation of low frequency noise and it is also often seen in
sound measurement equipment. When measuring suppressed weapons, the C-weighing
is often used to provide a better idea on the effects which a specific suppressor has on
the weapons sound signature. B-weighing plays a middle ground between A and C and
is somewhat rare in sound measurements. D-weighing can be seen from time to time but
is more closely related to the measurement of aircraft noise and it is not a relevant
filtering system in this case. [7]
4.1.
Testing equipment
Available measuring equipment form a significant factor in the accurate measurement
of sound characteristics of silenced weapons. Normal unsuppressed hand weapon sound
characteristics are measurable with a large variety of equipment as long as they can
withstand the sound pressures involved.
A suppressed weapon signature on the other hand forms an additional challenge
to measuring. The peak sound pressure of a suppressed weapon is considerably shorter
39
in duration than that of an unsuppressed one. This requires that the used measuring
equipment has to be fast enough to react to the short impulse with sufficient accuracy.
The reaction time is best represented by the rise time capability. A rise time of
20 microseconds for the whole system is deemed as necessary to achieve reliable
readings. Slower rise time means that the peak pressure is long past the microphone
before the system even responds and this might yield as large as a 10dB difference in
readings. [7]
4.2.
Suppressor test
Originally this study was intended to contain several live firing tests which would
concentrate on the performance of materials under firearm suppressor conditions.
Unfortunately only one test was managed to carry within the time limits of the work.
4.2.1.
Real fire test
The first prototype was tested in an indoor shooting range. The range was built within a
cave and the range itself was long and narrow. The test arrangements were not meant
for measuring the sound damping abilities of the suppressor, but to test the endurance of
the carbon fiber outer shell. The suppressor and the weapon were fixed to a shooting
bench and closed into a chamber where the weapon could be discharged and observed
behind a safety glass.
The first prototype raised suspicion on whether it could withstand normal
firearm conditions as the epoxy matrix is not a material commonly seen in high
temperature and pressure applications. The aim of the first test was to find out whether
carbon fiber composite could be used as outer shell material in rifle caliber suppressors.
In order to test this the suppressor was first subjected to conditions which a normal
hunting weapon suppressor might face and after that to conditions which far exceed
those circumstances to find out how much the suppressor would really endure. The first
prototype was attached to a test weapon, which in turn was attached to a test firing
bench. The first test was conducted with a RK62 7.62 caliber assault rifle. The test
began with two groups of four single rounds and one group of ten single rounds, which
were followed by the visual confirmation of the outer shell integrity. A hunting weapon
suppressor might go through such conditions during zeroing or target practice with the
exception that hunting weapons rarely hold clips larger than 5 shots.
As consecutive single shots proved to have no visual effect on the carbon fiber
material it was decided that the material integrity and endurance would be tested under
rapid fire circumstances. As normal live fire circumstances did not show any
deficiencies in the suppressor, the test continued by testing abnormal circumstances
such as continued short burst fire. A clip of 30 rounds which was fired in quick bursts of
two to three shots proved that the outer casing of the suppressor was not sufficiently
40
tight as propellant gas leaked between the top cap and the outer shell. This leakage was
strong enough to cause damage to the carbon fiber composite shown in Figure 26.
Figure 26 Damage caused to the suppressor outer shell by propellant gas leak between the end cap and
outer shell.
The prototype was repaired by cutting off the damaged segment of the carbon
fiber tube and reassembling the suppressor. As the gas leak was most likely a result of
poor fitting between the outer casing and end cap and not due to material deficiency, the
full thermal stress endurance potential of the material was still a question. Therefore test
continued by subjecting the suppressor to further rapid fire. After 60 rounds of short
bursts the suppressor casing started to show signs of alteration. The gas leakage did not
manifest itself again but the material darkened which was deduced as a sign of the
carbon fiber epoxy matrix reaching its thermal damage temperature. After this first
visual alteration the suppressor still withstood about 35 rounds of rapid fire in short
bursts before rupturing. The suppressor burst along a fairly straight line on the carbon
fiber outer shell and flew several meters from the weapon. The rupture is shown in
Figure 27. The first test proved the ability of the carbon fiber composite to withstand a
lot more heat strain than the normal use as a hunting suppressor would require.
41
Figure 27 The ruptured first prototype. The rupture progressed along a fairly straight line on the
suppressor outer shell.
42
5.
RESULTS AND DISCUSSION
The results of this study stand more on a theoretical foundation than on empirical
evidence as due to time issues only one live fire test could be accomplished. The results
of this test, however, are promising for carbon fiber composites and their use in
suppressors. The outer shell of the first prototype proved to be able to withstand a
considerable amount of heat before the structural integrity of the shell was compromised
sufficiently to cause rupture.
While the heat resistance was proved to be promising the test also revealed a
notable issue with the materials susceptibility to fraying under strong gas flow. This can
be countered by ensuring that the edges of carbon fiber parts are not subject to direct gas
flow or leaks or by providing some kind of protection against fraying.
Another issue which the test revealed is how any contact between the inner
metallic structure and outer shell increase the heat transfer to the carbon fiber
component. The steel baffles were designed to rest against the outer shell and during
testing the heat buildup was clearly more evident in areas which had a contact with the
baffles when compared to other areas of the outer shell. Even with a change of baffle
materials from steel to aluminum the issue would most likely prevail as aluminum has
even higher thermal conductivity than steel. Therefore the contacts between inner
metallic structure and any carbon fiber components should be minimized. Even if an
aluminum part might not have immediate contact with a carbon fiber composite material
prior to discharge, it is possible for the situation to change during firing with the high
thermal expansion of aluminum.
The first test concentrated upon the validity of carbon fiber as a suppressor
material in the outer shell of the prototype. The second step is to improve the internal
structure and find a construction that works with lighter materials like aluminum. These
modifications are easy to test in the same testing environment and with the same
equipment as used with the first test. For obtaining more complete results also different
caliber weapons should be used. The assault rifle cartridge is shorter than those used in
some hunting rifles and therefore the increased propellant gas pressure might cause
damaging issues in hunting rifles.
Finding a working structural solution may require several prototypes and tests
and even though the testing environment used in the first test was not optimal for
measuring the effect the suppressor has on shot signature, a rudimentary approximation
of the effect is still possible. A single decibel measurement device could be fitted inside
the testing area to measure the values from an unsuppressed discharge, from the
prototype suppressors and from a commercial competitor. By comparing these values a
43
rude approximation can be derived whether the suppressor attenuates the weapon
signature to any extent. The close walls and strong echo of the shooting range make it
almost impossible to measure the sound signature any further.
When the suppressor design is sufficiently advanced and the sound attenuation
requires more attention, a more open testing environment is required. An open field
shooting range would be near ideal to provide comparable values and microphone
placement should follow Figure 24. It is also worth considering that the suppressors
might work quite differently when used with different weapons and different cartridges
even if the caliber remained the same. Therefore the future tests would benefit from
several different testing weapons and from the use of multiple different cartridges. The
subsonic ammunition should also be included, as it is used in some cases to control the
weapon signature even further.
44
6.
CONCLUSIONS
The aim of this study was to improve an existing suppressor product by finding
alternative materials which are suitable for suppressor conditions and provide some
advantage over the previously used steel. The theoretical process found several possible
alternatives which would be worth of a closer look. Polymer matrix carbon fiber
composite and anodized aluminum turned out to be the most interesting alternatives. As
the company Oricopa Oy already has contacts with carbon fiber composite
manufacturers, they would also form the most available options to test in a prototype.
The carbon fiber composite was proved to have potential as a possible replacement for
steel when used as the material for the outer casing of the suppressor.
The research did not fulfill all of its goals as the time issues did not allow the
testing of anodized aluminum in the internal structure and possible combinations of
aluminum parts with carbon fiber composite components. Theoretical designs were
created for possible future testing and prototype construction.
As carbon fiber composites are somewhat a rarity in rifle caliber suppressors
these results find promise for the material and encourage for further research on the
subject. Light and strong suppressor designs with easy maintenance and reasonable
manufacturing costs would be welcomed among hunters and sport shooters.
45
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